1. Field of the Invention
The present invention relates to a semiconductor laser element having an optically-nonabsorbent window structure at the end facets.
2. Description of the Related Art
Maximum optical output power of semiconductor laser elements is known to be limited by catastrophic optical mirror damage (COMD), wherein end facets are damaged by a cycle in which currents generated by optical absorption at the end facets raise the temperature at the end facets, the raised temperature reduces the semiconductor bandgaps at the end facets, and therefore the optical absorption is further enhanced. The optical output power level, at which the COMD occurs (COMD level), decreases with degradation of the end facet caused by aging. In addition, there are cases in which the semiconductor laser suddenly shuts down. Accordingly, high output power and high reliability are known to be obtained by forming an optically-nonabsorbent window structure at the end facet of semiconductor laser element.
Kazushige Kawasaki et al. (xe2x80x9c0.98 xcexcm band ridge-type window structure semiconductor laser (1),xe2x80x9d Abstracts of the Spring Meeting of the Japan Society of Applied Physics, 1997, 29a-PA-19) disclose a semiconductor laser element which emits laser light in the 980 nm band and has a window structure at its end facets. The window structure is formed by injecting Si ions into near-edge regions of a ridge structure and disordering an In0.2Ga0.8As quantum well by thermal diffusion.
H. Horie et al. (in xe2x80x9cReliability improvement of 980-nm laser diodes with a new facet passivation processxe2x80x9d, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5 (1999), No. 3, pp. 832-838) disclose a semiconductor laser element having an internal current confinement structure. The semiconductor laser element comprises an InGaAs active layer, GaAs optical waveguide layers, AlGaAs cladding layers, and an AlGaAs current confinement layer. In addition, cleaved end facets are irradiated with Ar ions having energy not higher than 35 eV, and coated with silicon by vapor deposition. Then, AR/HR coatings are administered on the end facets by an ion assist vapor deposition method, where the average acceleration voltage for Ar ions is 110 eV. Thus, this semiconductor laser element can achieve high output power and reliability. Further, Horie et al. report that since the upper and lower optical waveguide of the semiconductor laser element are formed by GaAs, the crystal quality is maintained even if the temperature is raised and lowered during GaAs growth, and since InGaAs of the active layer can be grown in low temperatures, the crystal quality can be improved.
However, in the former report, there is a drawback that a process for injecting Si ion in the vicinities of the active layer is required and the fabrication process becomes longer. In addition, since a diffusion process is required, obtaining an accurate window structure is difficult. In the latter report, the low-energy ion acceleration requires expensive equipment, and the cost increases.
The present invention has been developed in view of the above circumstances. The object of the present invention is to provide a semiconductor laser element which is reliable in operation from low to high output power, having an optically-nonabsorbent window structure at its end facets.
According to the present invention, there is provided a semiconductor laser element having optical waveguide layers whose bandgap energies are greater than a bandgap energy of an active layer, comprising: a substrate of GaAs of a first conductive type; a lower cladding layer of the first conductive type, formed above the GaAs substrate; a lower optical waveguide layer of the first conductive type or an undoped type, formed above the lower cladding layer; a lower GaAs layer formed above the lower optical waveguide layer; an active layer of Inx1Ga1-x1As1-y1Py1 (0.49y1 less than x1xe2x89xa60.4, 0xe2x89xa6y1xe2x89xa60.1), formed above the lower GaAs layer; an upper GaAs layer formed above the active layer; an upper optical waveguide layer of a second conductive type or an undoped type, formed above the upper GaAs layer; an upper cladding layer of the second conductive type formed above the upper GaAs layer; and a contact layer of the second conductive type being formed above the upper cladding layer; wherein:
at least the upper GaAs and the active layer among the lower GaAs Layer, the active layer, and the upper GaAs layer, are formed in regions except at least a vicinity of one emission end facet of a resonator, and the upper optical waveguide layer is formed so that the end portions thereof bury the vicinity of the emission end facet.
Note that at the two end facets of the resonator, the vicinity of the end facets may be removed.
The contact layer of the second conductive type can be formed in a region except the vicinity of the end facets, and in this case, it is desirable that an insulative film having an opening for current injection is formed from the upper surface of the upper cladding layer of the second conductive type to the upper surface of the contact layer of the second conductive type, and an electrode is formed at least on the opening mentioned above. That is, the electrode may be formed on apart of the insulative film corresponding to the contact layer of the second conductive type, or on the entirety of the insulative film, so as to cover the opening.
A GaAs layer with a thickness of approximately 20 nm may be formed under the upper optical waveguide layer.
A semiconductor laser element according to the present invention may have a refractive index waveguide mechanism formed by a ridge structure. In addition, a semiconductor laser element according to the present invention may have a refractive index waveguide mechanism formed by an internal current confinement structure.
A semiconductor laser element according to the present invention may have a first etching block layer of the second conductive type of Inx9Ga1-x9P (0xe2x89xa6x9xe2x89xa61), a second GaAs etching block layer, a current confinement layer of the first conductive type of In0.5(Ga1-z4Alz4)0.5P (0xe2x89xa6z4xe2x89xa61), and a cap layer of an InGaP family crystal, lattice matched to GaAs, are formed in this order on the upper optical waveguide layer. There may be provided an opening with a width from approximately 1 xcexcm to approximately 4 xcexcm between the two facets of the resonator in the cap layer, the current confinement layer and the second etching block layer, and the upper cladding layer of the second conductive type and the contact layer of the second conductive type may be provided so as to bury the opening. Note that there may be another upper cladding layer made of the second conductive type Alz1Ga1-z1As (0.25xe2x89xa6z1xe2x89xa60.8) between the upper optical waveguide layer and the first etching block layer.
In addition, in a semiconductor laser element according to the present invention, the upper cladding layer of the second conductive type may be made of a first upper cladding layer of the second conductive type and a second upper cladding layer of the second conductive type. The first upper cladding layer of the second conductive type of Alz1Ga1-z1As (0.25xe2x89xa6z1xe2x89xa60.8), first etching block layer of the second conductive type of GaAs, a second etching block layer being made of Inx8Ga1-x8P (0xe2x89xa6x8xe2x89xa61), a current confinement layer of the first conductive type being made of Alz3Ga1-z3As (z1xe2x89xa6z3xe2x89xa60.8) and a GaAs cap layer may be formed above the upper optical waveguide layer in this order. An opening of from about 1 xcexcm to about 4 xcexcm width may be provided between the two facets of the resonator in the cap layer, the current confinement layer and the second etching block layer, and said opening may be filled in by the second upper cladding layer of the second conductive type with the contact layer of the second conductive type formed above the cladding layer.
In addition, in a semiconductor laser element according to the present invention, the upper cladding layer of the second conductive type may be made of a first upper cladding layer of the second conductive type and a second upper cladding layer of the second conductive type. The first upper cladding layer of the second conductive type of an InGaP family crystal, lattice matched to GaAs, a GaAs etching block layer, a current confinement layer of the first conductive type of In0.5(Ga1-z4Alz4)0.5P, and a cap layer made of an InGaP family crystal, lattice matched to GaAs may be formed above the upper optical waveguide layer in this order. An opening of from about 1 xcexcm to about 4 xcexcm width may be provided between the two facet of the resonator in the cap layer, the current confinement layer and the second etching block layer, and said opening may be filled in by the second upper cladding layer of the second conductive type with the contact layer of the second conductive type formed above the cladding layer.
Note that the xe2x80x9cInGaP familyxe2x80x9d is made of three elements; In, Ga, and P, where composition ratio of In and Ga is different.
Preferably, the optical waveguide layers are made of Alz2Ga1-z2As (0 less than z2 less than 0.8) or Inx2Ga1-x2As1-y2Py2 (0 less than y2xe2x89xa61, x2=0.49y2), lattice matched to GaAs. It is desirable that compositions are adopted so that the bandgap of the optical waveguide layer is greater than the bandgap of GaAs and smaller than the bandgaps of the cladding layers, and the refractive index of the optical waveguide layer is smaller than the refractive index of GaAs and greater than the refractive indices of the cladding layers.
A barrier layer of Inx4Ga1-x4As1-y4Py4 (0xe2x89xa6x4 less than 0.49y4, 0 less than y4xe2x89xa60.6), having tensile-strain and a greater bandgap energy than a bandgap energy of the active layer, may be formed adjacent to the active layer.
In addition, the cladding layer is of a composition that lattice matches with the substrate.
Defining CGaAs as the lattice constant of GaAs, ca as the lattice constant of an active layer, the strain amount of an active layer xcex94a is expressed as xcex94a=(caxe2x88x92CGaAs)/CGaAs. Defining cb as a lattice constant of a tensile-strain barrier layer, a strain amount of a tensile-strain layer xcex94b is expressed as xcex94b=(cbxe2x88x92CGaAs)/CGaAs. Further, defining da as a thickness of an active layer, db as a one-side thickness of a barrier layer, a layer composition of 0.25 nmxe2x89xa7xcex94ada+2xcex94bdbxe2x89xa7xe2x88x920.25 nm which does not spoil the crystallinity of the active area is preferable.
In addition, defining the lattice constant of a growing layer as c, lattice matching to GaAs means that an absolute value of a strain amount xcex94 is less than 0.005 where xcex94=(cxe2x88x92CGaAs)/CGaAs.
The first conductive type and the second conductive type have reversed polarities from each other. For example, if the first conductive type is p-type, the second conductive type is n-type.
Said xe2x80x9cvicinity of end facetxe2x80x9d is preferably a region of a depth of about 5 xcexcm to about 50 xcexcm towards the interior of the element from the end facet. If the depth is less than 5 xcexcm, an optically-nonabsorbent region can not practically be formed because of current spread, and is not preferable due to degradation of facets occurring because of heat. If the depth is greater than 50 xcexcm, the optical output power is undesirably reduced because of the optical loss.
According to the laser element of the present invention, optical absorption in the end facets can be eliminated even when the temperature of the end facets is increased under high output power, since the above mentioned window structure exists and the upper optical waveguide layer that has a greater bandgap energy than a bandgap energy of the active layer is filled in the vicinity of the end facets. Therefore, current caused by an optical absorption can be prevented and a heat generation at the end facets can be reduced. Thus, a significant improvement in the COMD level is obtained, and high reliability under high output oscillation can be obtained.
In the present invention, the active layer can be formed at low temperatures since GaAs layers are formed both above and below the active layer and GaAs can be grown in varying temperature conditions. Thus a high quality crystal can be obtained.
The refractive index waveguide layer can be formed accurately to the end facet because layers are flatly formed in the vicinity of the end facet in the same manner as the inside of the resonator.
The contact layer of the second conductive type is formed in regions except the vicinity of the end facet, and an insulative layer having an opening to inject current is formed at the region in the vicinity of the emission end facets from above the upper cladding layer of the second conductive type to above said contact layer of the second conductive type, and an electrode is formed at least on the opening. That is, in the vicinity of the end facets, the current injection is restricted because there is no layer that can have an ohmic contact with the electrode, thus an optical density can be reduced at the end facets. Therefore, it is possible to reduce heat generation in the vicinity of the end facets, and significantly raise the COMD level caused by thermo runaway. Consequently, reliability of the semiconductor laser element in high output power operations can be enhanced.
It is preferable to apply the present invention to a semiconductor laser element with a refractive index waveguide mechanism formed by a ridge structure, because a high quality oscillation light can be obtained up to high output power.
It is also preferable to apply the present invention to a semiconductor laser element with a refractive index waveguide mechanism formed by an internal current confinement structure because a high quality oscillation light can be obtained up to high output power.
Electric characteristics such as a threshold current and reliability can be improved by having a tensile-strain barrier layer because strain compensation is performed in the active layer.